The present invention relates to rare earth magnet production, a master alloy used for magnet production and master alloy production.
Rare-earth magnets of high performance, typically, powder-metallurgical Sm--Co base magnets having an energy product of 32 MGOe, have been produced on a large scale. However, a serious problem with these magnets is that the raw materials, Sm and Co, cost much. Of rare-earth elements, some elements of small atomic weight, e.g., Ce, Pr, and Nd occur more abundantly and cost less than does Sm. Fe is more inexpensive than Co. For these reasons, R--T--B base magnets (wherein T stands for Fe or Fe plus Co) such as Nd--Fe--B and Nd--Fe--Co--B magnets have recently been developed, and sintered magnets are set forth in JP-A-59-46008. Sintered magnets may be produced by the application of a conventional powder metallurgical process for Sm--Co systems (melting.fwdarw.master alloy ingot casting.fwdarw.ingot crushing.fwdarw.fine pulverization.fwdarw.compacting.fwdarw.sintering.fwdarw.magnet), and may easily achieve high magnetic properties as well.
Generally, a master alloy ingot produced by casting is made up of an R.sub.2 Fe.sub.14 B phase of ferromagnetism (hereinafter referred to as the main phase) and a non-magnetic, R-enriched phase (again called the R-enriched phase), and is of a structure in which the main phase forming crystal grains is covered with the R-enriched phase forming grain boundaries. The master alloy ingot is then pulverized or otherwise reduced to a grain diameter smaller than the crystal grain diameter until magnet powders are obtained. Each or the magnet powder is chiefly composed of a magnet grain including the main and R-enriched phases and a magnet grain consisting substantially of the main phase alone or, in other words, being free from the R-enriched phase.
The R-enriched phase is converted into a liquid phase to have an action on accelerated sintering, and plays a vital role in the generation of a magnet's coercive force. It is thus preferable that the structure and size of the master alloy ingot and the conditions for pulverizing it are optimized, thereby preventing the R-enriched phase from being locally present in the compact.
Because of some difficulty involved in obtaining fine crystal grains by casting, however, a single crystal grain is usually pulverized to a number of magnet grains. This results in the incorporation in the resultant magnet powders of a large amount of magnet grains containing no R-enriched phase in addition to R-enriched phase-containing magnet grains. Further, the R-enriched phases, because of being segregated, are caused to exist locally in the master alloy ingot. There is thus some considerable difference in the volumes of the R-enriched phases between magnet grains.
This results in the marked local presence of the R-enriched phases in the compact, and so there is a lowering of its ability to be sintered, failing to yield a sintered magnet having high residual flux density. In addition, a sintered magnet, if somehow obtained, has only a reduced coercive force, due to the local presence of the R-enriched phases therein. Due to difficulty involved in breaking up the main phases, on the other hand, the larger the crystal grains, the longer the time taken to pulverize them to fine magnet grains and so the larger the amount of oxygen incorporated in them, not only making it impossible to obtain any high residual flux density, but making the proportion of much-coarser grains too large to obtain any high coercive force.
In order to obtain a magnet having high residual flux density, it is required to reduce the proportion of the R-enriched phases in the magnet. However, using a composition having a low R content as the starting material results in the precipitation of .alpha.-Fe phases in the master alloy ingot. Partly because of a lowering of magnet properties due to the presence of .alpha.-Fe phases and partly because of difficulty involved in pulverization, usually, some form of solution treatment is applied to the master alloy ingot to reduce the proportion of .alpha.-Fe phases. However, the solution treatment, because of being carried out at an elevated temperature of about 900.degree. C. or higher for about 1 hour or longer, gives rise to main phase and R-enriched phase growths, making the dispersion of the R-enriched phases in the master alloy ingot more unfavorable.
When the R content is reduced and so the dispersion of the R-enriched phases gets worse, the ability of the compact to be sintered gets worse or there is a need of carrying out its sintering for a longer time, during which the crystal grains grow, failing to achieve any high coercive force.
In view of such situations as mentioned above, a primary object of the invention is to improve the coercive force and residual flux density of R--T--B base sintered magnets.